In the oil, gas, petroleum, and power industries, emergency shutdown of a process must be provided for under certain fault conditions.
An emergency shutdown (ESD) system is usually implemented by pneumatically controlled shut off valves, which generally remain open while the process is operating safely. These valves are usually only closed when an emergency shut down is required, or for maintenance. Often, processes operate for long periods of time, e.g., years, without shutting down. As the shutdown valves are operated infrequently, there is a high possibility that they will stick or freeze when the shutdown operation is required, thus resulting in a dangerous condition if an emergency shutdown has been requested.
The problem can be exacerbated by economic conditions which lead to a reduction in the frequency of shutdowns or turn-arounds. For example, in some operations a process may run continuously for one or more years without shutting down the process for maintenance.
State-of-the-art ESD systems, which control shut-off valves, have a number of features to detect plant or process failures and typically include redundancies for added reliability. However, such systems may not provide for the testing of shut-off valves themselves, other than full stroking the valve. However, the problem with full stroking or completely closing the valve is that it causes an undesirable disruption in the process. To alleviate the problem, partial stroke testing systems have been developed. In a partial stroke test (PST), a valve is partially closed in order to confirm that it is not stuck in an open position.
PST is not only applicable to safety related applications but can also be used to enhance the operation of the valve. For example, in many process applications, the chemical composition of the flowing stream can cause material to build up on the valve internal body and trim surfaces. Over time this build-up may cause the valve to “stick” in that position and not stroke. PST can be used to simply “exercise” the valve while allowing it to partially stroke, keeping the valve surfaces that are required to move free from material build up.
Many PST systems use mechanical hard stop devices which normally require a purpose-built actuator with integral manually engaged travel stops or add-on type manually engaged stops mounted as an interface between the actuator and the valve. These mechanical stops offer the benefits of hard travel stops to prevent spurious over travel and allow full actuator torque output to operate a valve experiencing stiction. However, they suffer from several disadvantages in that they require extensive operator training and procedures both for engagement and disengagement operations. Furthermore, they typically cannot be immediately disengaged should an ESD occur during partial stroke testing. This severely compromises safety.
Other common PST systems have no hard stops but rely instead on the careful release of air pressure to allow the spring inside of the actuator to move the actuator and valve to a desired partial stroke position which is chosen to both provide maximum valve motion without disrupting the controlled process. However, as only a small percentage of the air pressure can be released, the available torque or force output from the actuator is only a very small percentage of the actuator rated torque or force. As a result, a small change in the valve resistance to motion is sufficient to prevent the small actuator output to cause valve motion. In this situation, additional air must be released to develop sufficient actuator output, however, at the resulting pressure the actuator will cause excessive (spurious) valve travel and a resultant process disruption.
In order to prevent spurious motion, designers of such PST systems program pressure and time limits so that if either is exceeded the PS test is aborted. Thus the user has what is called a false failure whereby maintenance must be performed to determine the cause of the failure. Too often nothing is found other than a slight resistance to valve motion. As the process must be shut down for this maintenance action, the PST system causes the very process disruption that it was designed to prevent.
An ideal PST system would eliminate the need for manual engagement while also eliminate the possibility of false failures.